1. Technical Field
This invention relates in general to telecommunications and, more particularly, to shared protection architectures.
2. Description of the Related Art
Over the last decade, the importance of telecommunications has increased dramatically. In order to accommodate the vast amount of information passed over telecommunications systems, such as the Public Switched Telephone Network (PSTN) and data networks, copper wires are being replaced with optical fibers, which are capable of carrying significantly more information.
A single fiber may transmit information over many different frequencies or “channels” using DWDM (dense wavelength division multiplexing) techniques. Improvements in fiber technology and electronics are increasing the number of channels that may be distinguished over a fiber and, thus, the amount of information that may be passed by a single fiber.
Increases in information bandwidth over a fiber, however, increase the importance of providing mechanisms to bypass failures in the network, until the failure can be corrected. Common failures include, for example, fiber breakages (typically caused by construction activities inadvertently cutting a fiber), fiber disconnects caused by accidents in the central office, and network element failures, such as laser failures.
In order to maintain communications in spite of a failure, ring architectures are often used. In a ring architecture, a series of network elements are connected in a ring, such as shown in FIG. 1. Each ring 10 has multiple network elements 12 coupled to one another to form a closed loop. Typically, there are four fibers 14 connecting adjacent network elements 10—two working fibers and two protection fibers, although other configurations are possible. The working fibers (W) carry traffic between adjacent nodes. Protection fibers (P) are available to carry traffic in the event of a working fiber failure. The protection fibers also convey control information between network elements; when not being used for traffic, the protection fibers may carry low-priority interruptible traffic. As shown in FIG. 1, network elements 12 may be shared between different rings.
The ring architecture shown in FIG. 1a is a very simple architecture. In many circumstances, multiple rings 10 may connect various network elements 12 as shown in FIG. 1b. Failures of a working fiber in any of the rings 10 may cause protect lines in multiple rings to be used.
FIG. 2a illustrates one prior art method of circumventing a failure of a working fiber W. In this embodiment, a ring 10 having five network elements 12 (referenced individually as network elements 12a-12e) has a broken working fiber W between network elements 12c and 12d. For purposes of illustration, only one working fiber W and one protection fiber P is shown, it being understood that a similar pair of working and protection fibers are used for traffic in the opposite direction. To pass traffic between network elements 12c and 12d, network element 12d connects the working lines 16de to protect lines 18cd and network element 12c connects working lines 16bc to protect lines 18cd. In other words, traffic that would normally be routed over working lines 16cd is switched to the associated protect lines 18cd. This is referred to as a “span” switch.
FIG. 2b illustrates a situation where both the working and protection lines have failed between network elements 12c and 12d. In this case, a “ring” switch is implemented where working line 16de is rerouted to protect line 18de and working line 16bc is rerouted to protect line 18bc. Accordingly, the remaining viable protect lines all carry traffic. Every network element can still communicate with all the other network elements 12 on the ring.
FIG. 3 illustrates an architecture wherein two rings 10a and 10b share a path between network elements 12a and 12b. In WO 99/23773 (PCT/IB98/01955) to Elahmadi et al, the use of a single physical span between these two network elements is proposed. This single span provides protection for two rings 10a and 10b. A failure on either ring can be remedied by using the shared protect line 18ab to carry traffic. This architecture reduces costs, which can be significant if the distance between the shared network elements is long (or there are other infrastructure costs involved), but increases the chance of a traffic outage if another failure occurs on another ring. With more complicated ring architectures, with multiple physical rings using a common span, the chances of multiple concurrent failures on multiple rings can become significant.
Therefore, a need has arisen for a method and apparatus for sharing protect lines while maintaining reliability of the network.